Ibrahim

A digital thermometer at both ends so you have an actual gradient rather than one number, and look at the water dish. Chlorinated tap water in the soak dish is a common and invisible stressor, so dechlorinate it and change it often.

Two other normal causes before anybody panics. Background and light: they shift tone toward their surroundings, so a dark background and brown branches make a frog look darker than he is. And just before a shed the skin dulls, and since they generally eat the shed you often never see the evidence.

The stress list is where I would look if he stayed dark around the clock: temperature wrong either way, humidity too low, too much handling, bright light with nowhere to hide.